The present invention relates to an improved insulation system for a reactor used to react chlorosilanes and hydrogen gases. The system described comprises an inner graphite radiant heat shield and an outer carbon-based rigid felt insulation of high density.
The reaction of hydrogen with chlorosilanes requires a temperature in the range of 500.degree. C. to 1100.degree. C. The kinetics of these reactions are improved at higher temperatures. However, the ability to achieve and maintain higher temperatures within the reactor is limited by the economics required to provide the additional radiant heat and the ability of the reactor to tolerate the additional heat. The preferred method to maintain higher temperatures within the chlorosilane and hydrogen reactor is by improved insulation. However, increased insulation alone is not enough; the insulation must be capable of withstanding the higher temperatures created.
Insulations fabricated from carbon and graphite are known to have high heat stability. However, these carbon based insulation materials, when used in a chlorosilane and hydrogen reactor, have been found to react with hydrogen to form methane and with chlorosilanes to form silicon carbide. These reactions reduce the insulating capacity of carbon based insulation as well as reduce the structural integrity.
The three generally accepted modes of heat transfer through insulation are by electromagnetic radiation, conduction, and convection. Electromagnetic radiation heat transfer predominates at temperature above about 1000.degree. C., but at temperatures below about 1000.degree. C. conduction and convection become increasingly important as the mode of heat transfer. In general, density and reflective properties make a material effective against electromagnetic radiation heat loss. However, as the density of a material increases the heat loss due to conduction increases. Therefore, to reduce heat loss as a result of conduction through the materials, insulations are typically made of low density felts.
Current art suggests that an improved high temperature insulation can be achieved by combining a series of radiation shields with a low density, flexible, felt insulation. The number of radiation shields required to drop the temperature to a point below which radiation predominates as the principal form of energy loss can be estimated by standard means. C. K. Crawford, J. Vac. Sci. Technol. 9:23, 1972. The art teaches a low density felt can be used in areas of a furnace where conductive and convective energy becomes of increasing importance.
It is known that the thermal conductivity of hydrogen is greater than that of air. Edstrand, E. W., Evolution and Applicability of High Temperature Electric Heating Fiber Modules, Industrial Heating, November, 1986, teaches that, in insulation made from ceramic fiber, the presence of hydrogen gas can increase heat loss dramatically. Edstrand suggests improved insulating capabilities can be realized when the ceramic fiber insulation has a density of 12 lb/ft.sup.3 or more. Edstrand does not provide any information on the effect of the presence of hydrogen gas on insulation made from carbon based materials.
The present invention provides a thermal insulating system for a chlorosilane and hydrogen reactor. It was recognized during the development of this invention that minor amounts of chlorosilanes and hydrogen gases may escape from the reactor and come in contact with the insulation. The insulating system design of the present invention reduces the impact of the reactions of the escaped chlorosilanes and hydrogen gases with carbon-based felt insulation. Also, the thermal insulating system design recognizes the impact of hydrogen gas on heat transfer through a carbon-based felt and reduces this impact. The thermal insulation system design allows the chlorosilane and hydrogen reactor to be operated at higher and more efficient temperatures for longer periods of time.